1. Field of the Invention
The present invention relates to a vacuum heat treatment furnace, and more particularly to a vacuum heat treatment furnace structured so as to prevent a deterioration due to a carburizing and increase an efficiency of thermal energy with respect to a subject or a work to be treated in a vacuum heat treatment, a vacuum carburizing treatment and the like, and a method of and apparatus for measuring a carbon concentration in an atmosphere having a reduced pressure.
2. Description of the Prior Art
A method of carburizing an iron and steel material under reduced pressure conventionally includes a vacuum carburizing method carried out by introducing a hydrocarbon series gas within a furnace, in which a carbon material is frequently employed for a heat insulating material and a heater as furnace constituting members, and a heat resisting steel, a heat resisting cast steel, a carbon series material or the like is employed for a hearth rail for supporting the subject to be treated and a hearth roller for transferring. Further, the vacuum heat treatment furnace is of a type in which a furnace wall is water cooled and a type in which the furnace wall is not water cooled by making the heat insulating material thick so as to increase a heat insulating property, in view of a working environment and the heat insulating property.
However, in the case that the iron series material (a hard carburized material containing a high Ni high Cr material) is used for the furnace constituting material in a high temperature range, the furnace constituting material is also carburized at the same time when the subject to be treated is carburized, in the vacuum carburizing treatment, so that the material becomes brittle and is broken. Further, even in the case of making the heat insulating material thick without water cooling the furnace wall so as to increase the heat insulating property and reduce an amount of heat radiation, for the purpose of reducing the thermal energy, a large amount of heat reserve of the heat insulating material is required, so that the thermal energy required for increasing the temperature is increased and a lot of time is also required for reducing the temperature.
Further, as a means for directly measuring a carburizing within a heat treatment furnace or a carbon concentration of an anti-oxidized atmosphere, a steel wire method is shown, for example, in Japanese Patent Publication No. 45-35411. Further, with respect to a holding rod for holding the steel wire for measuring the carbon concentration, for example, there is a structure shown in Japanese Utility Model No. 2586972.
However, the measurement in accordance with the steel wire method is carried out by inserting the steel wire for measuring the carbon concentration into an atmosphere normally equal to or more than atmospheric pressure, and it is hard to insert the steel wire for measuring the carbon concentration in the atmosphere having reduced pressure. Further, the vacuum carburizing under reduced pressure corresponds to a treatment in which a carbon is directly carburized into the steel, and it is not carried out to control and measure the carbon concentration in the atmosphere within the furnace.